1. Field of the Invention
This invention relates to light valves, and more particularly to a light valve with rotatable vanes controlled by separate integral servo motors that provide a fast response time for the light valve. One use of the light valve is in high speed film printing in which printing speed is increased over systems using slower conventional light valves.
2. Description of the Prior Art
A light valve is a device that varies the size of an aperture through which a light beam passes. A common use of light valves today is in film printing. The light valve is adjusted to control the cross-sectional size of the light beam passing through it for adjusting the exposure in accordance with the light levels of scene changes in the film being printed. Typically, separate light valves are used for each of three principal colors in the film printing process, and the exposures from each light valve are independently controlled.
A common form of light valve includes a pair of vanes carried on a pair of rotating shafts. The vanes protrude into the path of the light beam, and the shafts are rotated to move the vanes toward or away from each other to form a variable width optical slit for controlling the amount of light passing through the vanes. Usually, the shafts are counter-rotated relative to each other and through approximately the same phase angle so the center of the aperture formed by the vanes does not appreciably shift laterally during operation of the light valve.
Light valves of various forms have been developed in the past for controlling the phase angle of the shafts that rotate the vanes. These have included use of a stepper motor, or means for rotating the shafts against stops which have been preset prior to each new aperture change. When the shafts are rotated by a common motor, mechanical coupling means such as linkages, gears, or the like are used to rotate both shafts simultaneously through the same angle. Such mechanical coupling increases the inertia of the moving parts which, in turn, slows down the response time of the light valve. Also, it is difficult to avoid backlash and added elasticity which can cause oscillations in the mechanical system that controls the vanes. Moreover, these problems are amplified when attempting to speed up the response time of the light valve. For instance, in the past, the fastest light valves with mechanical coupling have had a response time on the order of about 5 milliseconds. In a film printing system, "response time" is the time required for the vanes to assume a new position to produce a new sized aperture for each scene change in the film. Present light valves with their connecting linkages and gearing have had endless oscillation problems when operated at higher speeds. Film printing speed with such divices has been limited to about 600 feet per minute.
There is a need to provide a film printing system that can operate at higher speeds. The slow response time of the light valves with mechanical coupling as used in previous film printing systems has been a principal obstruction to higher speed printing.